The present invention relates to a lamp driving apparatus and a method for driving a lamp having a filament such as an incandescent bulb.
In general, a lamp driving apparatus capable of controlling a lamp driving includes a controller for producing a lighting/non-lighting control signal in accordance with an instruction signal that is output from an instruction device having a switch, a sensor, or the like and serves as an instruction to turn on or off a lamp and a driving section for supplying power to the lamp in accordance with the lighting/non-lighting control signal. When receiving an instruction signal as an instruction to turn on or off the lamp, the controller causes the driving section to apply or not to apply a voltage to the lamp.
Among such lamp driving apparatus is one capable of dimming auxiliary lamps of a vehicle (i.e., lighting the lamps at low luminance) by PWM (pulse width modulation) control in accordance with the situation (refer to JP-UM-A-5-26629 (pages 6–9, FIG. 1), for example). Different from lamp driving that merely applies a DC voltage to a lamp Hereinafter referred to as “DC lamp driving”), the lamp driving by PWM control (hereinafter referred to as “PWM lamp driving”) can adjust the brightness of a lamp finely because it can vary the duty cycle of a pulse voltage that is applied to the lamp. As is well known, it goes without saying that lamp driving apparatus capable of performing PWM lamp driving can also perform DC lamp driving.
Incidentally, vehicles (what is called 14-V vehicles) that incorporate a power section (rated output voltage: DC 12–14 V) having a 14-V alternator and a 12-V battery that can be charged and discharge are known as common vehicles. Lamp driving apparatus of such 14-V vehicles perform DC lamp driving by applying DC voltages of 12–14 V to various lamps while their driving section is supplied with power from the power section (rated output voltage: DC 12–14 V). Depending on the lamp driving specification, the lamp driving apparatus also perform PWM lamp driving by applying pulse voltages having a DC component of 12–14 V.
In recent years, various efforts have been made to develop fuel-efficient, high-voltage vehicles (what is called 42-V vehicles) that incorporate a power section (rated output voltage: DC 36–42 V) having a 42-V motor/generator and a 36-V battery that can be charged and discharge. At present, for a transition from 14-V vehicles to 42-V vehicles, to use, also in 42-V vehicles, various general-purpose lamps that are used in 14-V vehicles that are advantageous in cost, lamp driving apparatus for driving those lamps for 14-V vehicles are being studied. Such lamp driving apparatus perform PWM lamp driving by applying a pulse voltage having a DC component of 36–42 V to various lamps while their driving section is supplied with power from the power section (rated output voltage: DC 36–42 V). That is, the PWM lamp driving properly adjusts (i.e., decreases) the power that is supplied to the 14-V lamps by applying pulse voltages having a proper pulse width to the lamps.
However, in the above PWM lamp driving, in the case where a plurality of lamps are lit during a night drive or the like, if switching signals (i.e., PWM signals) are applied, with the same timing, to switching elements (e.g., transistors or FETs) that are provided for the respective lamps, large rush currents flow from the power section into the filaments of the lamps instantaneously at the start of application of the voltages, as a result of which a large noise pulse (i.e., switching noise) appears on the power line. This is a serious problem particularly in 42-V vehicles in which the output voltage of the power section is high.
On the other hand, to solve the above problem, a lamp driving apparatus has been proposed which drives a plurality of lamps for their switched lighting by supplying switching elements corresponding to the respective lamps with switching signals whose switching on-periods are shifted from each other (refer to JP-A-2001-239879 (pages 3–5, FIGS. 1–3), for example).
FIG. 3 is a circuit block diagram schematically showing a lamp driving apparatus disclosed in JP-A-2001-239879. FIG. 3 shows a related lamp driving apparatus that is equipped with a plurality of lamps 2a–2i (a headlamp (right) 2a, a headlamp (left) 2b, fog lamps (right/left) 2c, 2d, tail lamps (right/left) 2e, 2f, a direction-indicating lamp (right) 2g, direction-indicating lamp (left) 2h, backup lamps (right/left) 2i) that are provided on a vehicle, a power section (i.e., battery) 1 for supplying power for lighting the lamps 2a–2i, and an instructing member 5 having a plurality of lamp lighting instruction switches or the like for making instructions to light the respective lamps 2a–2i. This lamp driving apparatus is also equipped with a plurality of switching elements 3a–3g for lighting the respective lamps 2a–2i by supplying currents to those and a controller 7 for supplying corresponding ones of the switching elements 3a–3g with switching signals for lighting lamps that the controller 7 has been instructed to light by the instructing member 5. The lamps 2a–2i are right and left headlamps, right and left fog lamps, right and left tail lamps, right and left direction-indicating lamps, right and left backup lamps, etc.
In this lamp driving apparatus, when lighting instruction switches for some of the headlamps, fog lamps, tail lamps, direction-indicating lamps, backup lamps, etc. are turned on as instructions to turn on corresponding ones of the lamps 2a–2i, part of input signals 6a–6f corresponding to the turned-on switches are input to the controller 7. As shown in FIG. 4, the controller 7 supplies part of switching signals 4a–4g (headlamp (right) 4a, direction-indicating lamp 4b, fog lamp 4c, direction-indicating lamp 4d, headlamp (left) 4e, tail lamp 4f, backup lamp 4g) that are shifted from each other in time to part of the switching elements 3a–3g corresponding to the respective lighting-instructed ones of lamps 2a–2i and thereby lighting-drives those lamps sequentially. Since an event that voltages are applied to (part of) the lamps 2a–2i simultaneously is prevented, large switching noise is prevented from occurring on the power line.
However, in the above lamp driving apparatus, the controller 7 needs to supply switching signals to the switching elements 3a–3g for the respective lamps 2a–2i so as to shift the output time sequentially and continues such an output control. This complicates the configuration and the control procedure of the controller 7 of the lamp driving apparatus.
For example, where the above control is performed by software, interrupt routines are provided in the same number as the number of switching elements 3a–3g, which makes the control complex and may impair the reliability of the control. This may make it necessary to use an expensive microprocessor. Where the control is performed by hardware, it is unavoidable that the circuit configuration becomes complex and the number of parts used increases accordingly, resulting in cost increase of the entire system. The related lamp drive apparatus has those problems to be solved.